Wells in U.S. Pat. No. 940,292 discloses a process for treating nickel-copper mattes to recover nickel separately from the matte. This process involves leaching finely divided matte with hydrochloric acid solutions containing not more than about 25% hydrogen chloride to selectively dissolve the nickel values contained in the matte. Any copper dissolved is removed by fresh matte or by hydrogen sulfide which is a by-product of leaching. Nickel in a commercially acceptable form is recovered from the chloride solution by crystallization followed by hydrogen reduction or by precipitation of nickel ammine sulfate.
In U.S. Pat. No. 3,085,054 to Thornhill, there is disclosed a process for treating a nickel copper matte to recover metallic nickel and copper sulfide. The process involves the selective dissolution of the nickel values with a concentrated hydrochloric acid solution while leaving most of the copper values undissolved. After separation of the nickel chloride solution from the copper sulfide, the pregnant nickel chloride solution is oxidized with air or chlorine while adding further amounts of hydrogen chloride to oxidize ferrous iron to ferric iron and to form anionic chloride complexes of the various impurities. The thus treated pregnant solution is contacted with an anion exchange resin or a liquid extractant to extract those metals which form anionic chloride complexes. Additional amounts of hydrogen chloride are added to the purified nickel chloride solution to crystallize nickel chloride therefrom. Metallic nickel is recovered from the crystallized nickel chloride by electrolysis or by hydrogen reduction. When electrolysis is used to recover metallic nickel, gaseous chlorine is generated at the anode. In a separate operation, the gaseous chlorine is reacted with hydrogen and hydrogen sulfide generated during the dissolution of the matte to form hydrogen chloride which is recycled to both the dissolution and the nickel chloride crystallization steps. The liquor remaining after the nickel chloride crystallization step is treated to strip a portion of the hydrogen chloride therefrom and is then treated to remove lead. The lead-free liquor containing substantial amounts of nickel, e.g. more than 25 grams per liter, is heated and recycled to the matte leaching operation. When metallic nickel is recovered by hydrogen reduction the same general flowsheet is employed except that upon reduction of the nickel chloride with hydrogen a gaseous effluent containing hydrogen and hydrogen chloride is obtained which effluent must be treated to scrub the hydrogen chloride therefrom.
Although the process described by Thornhill is useful in producing a highly refined nickel product, there are certain inherent disadvantages. For example, as the process relies on the crystallization of nickel chloride for the ultimate recovery of nickel, it is necessary to recycle the mother liquor from which nickel chloride has been crystallized. Thus, substantial amounts of nickel are inventoried in the circulating mother liquor adding to the capital and operating costs. Another disadvantage of the process disclosed by Thornhill involves the recovery of metallic nickel. Whether metallic nickel is recovered by electrolysis or by hydrogen reduction, recovery of hydrogen chloride is complicated either by the fact that electrolysis generates gaseous chlorine which must be converted to hydrogen chloride, or in the reduction of nickel chloride with hydrogen both the hydrogen and hydrogen chloride must be recovered which entails not only an absorbing process for recovering hydrogen chloride but also a process for drying the hydrogen stream from which the hydrogen chloride has been absorbed. The conversion of gaseous chlorine to hydrogen chloride and the drying of the hydrogen stream from which hydrogen chloride has been removed entail additional capital and operating costs. Another disadvantage is the high cost of hydrogen.
Van Weert in U.S. Pat. No. 3,642,441 describes a process for hydrolyzing chlorides in fluidized beds and specifically refers to nickel chloride derived from the Thornhill process described above. Van Weert describes the hydrolysis of hydrated nickel chloride crystals, although he states that solutions can be similarly hydrolyzed. Although van Weert suggests his process may be capable of hydrolyzing metal chloride solutions, there is no suggestion that hydrochloric acid solution approaching its azeotropic composition and containing metal chlorides could be hydrolyzed by his process. Moreover, the van Weert process does not provide 100% hydrolysis of the metal chlorides and is silent on the recovery of the unconverted metal chlorides. Hydrolysis of nickel chloride crystals, besides presenting materials-handling problems, also presents control problems because the combustion of fuel must be carefully regulated to insure substantially complete combustion without generating an atmosphere containing such excesses of free oxygen which oxidize generated hydrogen chloride to gaseous chlorine which must be recovered with difficulty and with added cost. The presence of gaseous chlorine also creates severe corrosion problems.